Trailer Brakes Come On when Tow Vehicle Automatic Headlights Turn On

Question:

We have a horse trailer, the lights and brakes work fine until dark, when the automatic photo eye on the tow vehicle turns the vehicle headlight/running lights on-this causes the trailer brakes to be applied. The problem happens with different vehicles from different manufacturers.

asked by: Robert W

Expert Reply:

If you are using adapters to match the vehicle side connectors with the trailer side connector it could be that the adapter or the trailer side connector is not wired up correctly to match what is required. I have included a link to a help article that shows how vehicle and trailer side connectors should be pinned out so you can check yours.

If the vehicles have brake controllers it could be that the brake controller red wire is not attached to the correct brake switch wire. For hardwire applications that require splicing the brake controller red wire to a brake switch wire it is possible to attach it to the wrong brake switch wire. This is because a brake switch can have more than one wire that has power when the brake pedal is pressed. If the wire being used also has power when the headlights, automatic lights, or turn signals are on then it is not the correct wire.

If you can tell me the years, makes, and models of the vehicles in question I can determine if there are any other known issues with those vehicles.

It is also possible though unlikely that all of the vehicles in question have running light circuits in the tow package wiring that are shorting out to the brake output circuit (or if it is one trailer being towed with multiple vehicles it could be the trailer side). Make sure that the vehicle side connectors and trailer side connector are all clean and free of corrosion inside where the wires attach and outside on the pins.

expert reply by: Michael H